Several highly publicized bee die-offs have increased concern for the health of Oregon's bee populations, prompting investigations and the establishment of a legislative task force to examine pesticide use and improve pollinator habitat in the state.

A collapse in bee population could destabilize food supplies, as about a third of all the food we eat is dependent on bee pollination. In Oregon, commercial and wild bees provide an estimated $600 million in annual agricultural value.

And while pesticides have been implicated in several recent Oregon incidents, a number of other factors -- including starvation -- may be contributing to population declines in Oregon and across the country.

Pesticides implicated in bee deaths

Two recent incidents of mass bumblebee die-offs have been investigated by the Oregon Department of Agriculture and were found to be the result of a pesticide containing the neonicotinoid compound imidacloprid.

Beekeepers experiencing unusual colony losses should contact the local beekeeping association or the Oregon State Beekeepers Association. If you suspect pesticides are involved, please contact the ODA.

Last summer's high profile bee die-off in Wilsonville prompted the ODA to change the labeling on pesticides containing neonicotinoid ingredients, in particular the compounds imidacloprid and dinotefuran.

As of 2014, containers of pesticides containing these "neonics" bear a label prohibiting use on linden trees, basswood and other trees in the Tilia genus. These trees produce a mild natural toxin that is thought to amplify the pesticide's toxicity to bees.

But products made before 2014 still have the older labels, which allow for the pesticide to be used when plants are not in bloom.

Two weeks ago, Eugene residents found a thousand bumblebees dead and dying beneath blooming linden trees. ODA investigators determined that the chemical had been sprayed on the trees, in violation of label restrictions. The company responsible had their license suspended and may face fines, said ODA spokesman Bruce Pokarney.

Several days later, hundreds of dead bees were found in Beaverton, again under blooming linden trees. In this case, imidacloprid had been injected into the trees in May, prior to blooming, in apparent accordance with the restrictions noted on the pre-2014 label.

In response to these incidents, the ODA issued temporary restrictions June 26 prohibiting all use on linden and Tilia species, regardless of label, while ODA investigates and determines future regulatory actions.

Colony die-offs likely due to starvation

Recent reports of dead and dying beehives in the Portland area offer a deeper mystery.

Beekeeping hobbyist Dena Rash Guzman of Sandy despaired upon finding two of her eight beehives collapsed, with inches-deep layers of dead honeybees littering the bottom of the hives.

A number of other beekeepers in the area have reported similar symptoms.

Isaak Stapleton, investigator for the ODA pesticides program, and Dewey Caron, affiliate faculty in Oregon State University's horticulture department, visited Guzman's farm and several others to examine the afflicted hives.

Among several stress factors, Caron noted that the affected hives showed very little stored honey and pollen. "It could fairly be said that the colonies were living on the edge," Caron said, with worker bees collecting just enough food each day with no backup reserves.

While hive starvation is not uncommon in winter when food resources are scarce, it can also happen in seasons of abundance.

In blooming meadows of early summer, a bee colony with sufficient reserves can prosper and grow to a healthy population of 60,000 to 80,000. But for a colony on the edge, a few days of cool weather, such as those preceding the recent Portland-area hive deaths, could prevent bees from foraging and could tip the colony into starvation.

Stapleton sent samples of dead bees to the OSU honeybee lab to be tested for presence of pesticides. While unwilling to rule out pesticides until test results are available, Caron said the evidence supported stress and starvation as a more likely cause of the sudden hive losses.

Bee declines and the task force

Caron, a longtime beekeeper, and OSU entomologist Ramesh Sagili both contribute data to the Bee Informed Partnership, a nationwide collaboration of research labs focused on declining bee populations. According to the partnership, over the past six years, Oregon beekeepers have lost on average 22 percent of their colonies each winter.

While Oregon's colony losses are markedly lower than the national average of about 30 percent, they are still significantly higher than the 15 percent that beekeepers report as "acceptable."

Despite annual losses, Sagili notes that the overall number of hives has remained fairly consistent. Beekeepers can repopulate a lost colony by splitting a healthy hive, though the process is costly in time and money, putting economic pressure on Oregon's commercial beekeepers.

Sagili emphasizes that multiple stress factors add up to decreased colony health: poor nutrition due to extensive mono-crop agriculture and habitat loss; migratory stress as colonies are moved from crop to crop and state to state; and disease, fungus and parasitic Varroa mites.

Chronic low-level exposure to pesticides, especially neonicotinoids, may harm bees' ability to navigate while foraging and increase susceptibility to other stressors.

On Monday, the Oregon Task Force on Pollinator Health, created in response to last summer's bee die-offs, held its first organizational meeting.

The task force, which includes representatives from the state legislature, nurseries, industry, beekeepers and environmental groups, will meet several more times to investigate such items as best practices in pesticide use and how to increase pollinator habitat.